diff --git a/app/models/comment.rb b/app/models/comment.rb
index 7a373b0068..7b882c3178 100644
--- a/app/models/comment.rb
+++ b/app/models/comment.rb
@@ -37,6 +37,12 @@ class Comment < ApplicationRecord
                  instance_accessor: false,
                  default: DEFAULT_CREATION_RATE_LIMITS
 
+  cattr_accessor :old_age_in_days,
+                 instance_reader: false,
+                 instance_writer: false,
+                 instance_accessor: false,
+                 default: 30
+
   strip_attributes allow_empty: true
 
   belongs_to :user,
@@ -52,7 +58,7 @@ class Comment < ApplicationRecord
 
   # validates_presence_of :user # breaks during construction of new ones :(
   validate :check_body_has_content,
-           :check_body_uses_mixed_capitals
+           :check_body_uses_mixed_capitals, unless: :hidden?
 
   scope :visible, -> {
     joins(:info_request).
@@ -60,6 +66,8 @@ class Comment < ApplicationRecord
         where(visible: true)
   }
 
+  scope :hidden, -> { where(visible: false) }
+
   scope :embargoed, -> {
     joins(info_request: :embargo).
       where('embargoes.id IS NOT NULL').
@@ -79,6 +87,15 @@ class Comment < ApplicationRecord
 
   default_url_options[:host] = AlaveteliConfiguration.domain
 
+  def self.erase_old_hidden(editor: User.internal_admin_user)
+    old_hidden = hidden.where('updated_at > ?', old_age_in_days.days.ago)
+    reason = "Hidden for longer than #{old_age_in_days} days"
+
+    old_hidden.find_each do |comment|
+      comment.erase(editor: editor, reason: reason)
+    end
+  end
+
   # When posting a new comment, use this to check user hasn't double
   # submitted.
   def self.find_existing(info_request_id, body)
@@ -194,6 +211,11 @@ def hide(editor:)
     end
   end
 
+  def erase(**kwargs)
+    return false unless hidden?
+    Comment::Erasure.new(self, **kwargs).erase
+  end
+
   def cached_urls
     [
       request_path(info_request),
diff --git a/app/models/comment/erasure.rb b/app/models/comment/erasure.rb
new file mode 100644
index 0000000000..fc8e5e24be
--- /dev/null
+++ b/app/models/comment/erasure.rb
@@ -0,0 +1,43 @@
+class Comment::Erasure
+  def initialize(comment, editor: User.internal_admin_user, reason:)
+    @comment = comment
+    @editor = editor
+    @reason = reason
+  end
+
+  def erase
+    ActiveRecord::Base.transaction do
+      erase_comment
+      erase_comment_events
+    end
+  end
+
+  protected
+
+  attr_reader :comment, :editor, :reason
+
+  private
+
+  def erase_comment
+    event_params = {
+      comment_id: comment.id,
+      editor: editor.url_name,
+      reason: "Erased: #{reason}"
+    }
+
+    comment.update!(body: '')
+    comment.info_request.log_event('erase_comment', event_params)
+  end
+
+  def erase_comment_events
+    comment.info_request_events.edit_comment_events.find_each do |event|
+      params = event.params.dup
+
+      params.each do |key, _|
+        params[key] = "[ERASED]" if key =~ /body/
+      end
+
+      event.update(params: params)
+    end
+  end
+end
diff --git a/app/models/info_request_event.rb b/app/models/info_request_event.rb
index dc25750a7b..0bf5101e1e 100644
--- a/app/models/info_request_event.rb
+++ b/app/models/info_request_event.rb
@@ -35,6 +35,7 @@ class InfoRequestEvent < ApplicationRecord
     'edit_comment', # comment edited (in admin interface)
     'hide_comment', # comment hidden by admin
     'report_comment', # comment reported for admin attention by user
+    'erase_comment', # comment is erased
     'report_request', # a request reported for admin attention by user
     'destroy_incoming', # deleted an incoming message (in admin interface)
     'destroy_outgoing', # deleted an outgoing message (in admin interface)